Substrate for magnetic disk

ABSTRACT

A method for manufacturing a substrate for a magnetic disk, including the steps of (a) polishing a substrate with a polishing composition A containing alumina abrasives having an average particle size of from 0.05 to 0.5 μm, and an oxidizing agent, and (b) polishing the substrate with a polishing composition B containing silica particles having an average particle size of from 0.005 to 0.1 μm; a substrate for a magnetic disk, obtainable by the method for manufacturing a substrate for a magnetic disk; and a substrate for a magnetic disk having the following surface properties of a long-wavelength waviness of 0.05 nm or more and 0.3 nm or less, and an AFM surface roughness of 0.03 nm or more and 0.2 nm or less. The substrate for a magnetic disk may be suitably used in the manufacture of a hard disk having a high recording density. Especially, a hard disk having a recording density of 50 G bits or more per square inch may be industrially manufactured.

FIELD OF THE INVENTION

The present invention relates to a substrate for a magnetic disk whichcan be used, for instance, for a hard disk having a high recordingdensity, and a method for manufacturing the substrate.

BACKGROUND OF THE INVENTION

In order to have a smaller unit recording area and promote higherstorage capacity, hard disk drives having lower flying height of themagnetic head have been desired. In order to lower the flying height ofthe magnetic head, the surface smoothness of the hard disk substrate hasbecome increasingly important. So far, the surface smoothness has beenevaluated by surface roughness as determined by a profilometer having adiameter of 0.2 μm and a cut off of 25 μm (wavelength: 0.2 to 25 μm),and microwaviness as determined by cut-off of 800 μm (wavelength: 0.2 to800 μm), and reduction in the surface roughness and the microwavinesshave been tried.

As a method for manufacturing a substrate in need of surface smoothnessas evaluated by the surface roughness and the waviness as describedabove, mechanical conditions such as control of the pore size of thepolishing pad, control of the hardness, and control of the polishingload or rotational speed during polishing have been studied. On theother hand, as the process for improving the polishing composition andthe polishing process, in Japanese Patent Laid-Open No. Hei 11-10492, amethod including the steps of carrying out a polishing process by aplurality of steps, polishing with a polishing composition containingmetal oxide abrasive grains having a particle size of from 0.3 to 5 μmbefore final polishing, and further polishing with a polishingcomposition containing a colloidal particle having a particle size offrom 0.01 to 0.3 μm has been studied.

SUMMARY OF THE INVENTION

The present invention relates to:

-   [1] a method for manufacturing a substrate for a magnetic disk,    including the steps of:-   (a) polishing a substrate with a polishing composition A containing    alumina abrasives having an average particle size of from 0.05 to    0.5 μm, and an oxidizing agent, and-   (b) polishing the substrate with a polishing composition B    containing silica particles having an average particle size of from    0.005 to 0.1 μm;-   [2] a substrate for a magnetic disk, obtainable by the method for    manufacturing a substrate for a magnetic disk of the above [1]; and-   [3] a substrate for a magnetic disk having the following surface    properties:    -   a long-wavelength waviness of 0.05 nm or more and 0.3 nm or        less, and    -   an AFM surface roughness of 0.03 nm or more and 0.2 nm or less.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have found that the method thus far does not havesatisfactory surface performance as a substrate for a magnetic disk fora hard disk having a high recording density, especially a high recordingdensity exceeding 50 G (giga) bits or more per square inch. The presentinvention relates to a substrate having a high surface smoothnessrequired for a substrate for a magnetic disk having a high recordingdensity, especially, for instance, having a high recording density of 50G bits or more per square inch, and a method for manufacturing thesubstrate. More preferably, the present invention relates to a substratefor a magnetic disk, for instance, having a polishing rate of apractical manufacturing level, and satisfying both the surface roughness(AFM surface roughness) having a short wavelength and waviness having along wavelength (long-wavelength waviness), and a method formanufacturing the substrate.

These and other advantages of the present invention will be apparentfrom the following description.

According to the present invention, a substrate having a high surfacesmoothness required for a substrate for a magnetic disk having a highrecording density, especially, for instance, having a high recordingdensity of 50 G bits or more per square inch can be obtained.Especially, there is exhibited an effect that a substrate for a magneticdisk, for instance, having a polishing rate of a practicallymanufacturing level, and satisfying both the surface roughness (AFMsurface roughness) having a short wavelength and waviness having a longwavelength (long-wavelength waviness) may be obtained.

1. Substrate for Magnetic Disk

The preferred substrate for a magnetic disk of the present invention hassurface properties of a long-wavelength waviness of 0.05 nm or more and0.3 nm or less, and an AFM surface roughness of 0.03 nm or more and 0.2nm or less.

In the present invention, it has been found that measures used inevaluating surface properties which have been so far generally used fora substrate for a magnetic disk such as a hard disk, such as surfaceroughness having a wavelength of 1 to 80 μm or so, as determined by“Tencor P12” (trade name, commercially available from KLA-Tencor) and“Talystep” (trade name, commercially available from Rank Taylor-HobsonLimited) or microwaviness having a wavelength of 1 to 500 μm, do notserve as satisfactory measures for surface properties of a substrate fora magnetic disk for a high recording density. It has been further foundthat a roughness in a short wavelength region (AFM surface roughness)and a waviness having a long waviness (long-wavelength waviness) arevery useful as evaluation axes for expressing surface propertiesrequired for a substrate for a magnetic disk for a high recordingdensity.

Specifically, the term “AFM surface roughness” as referred to herein isan average surface roughness (Ra) which can be determined with aninteratomic force microscope (AFM) at a wavelength as short as 10 μm orless, and the term “long-wavelength waviness” as referred to herein isan average surface height obtained from a waviness curve having awavelength component of 0.5 to 5 mm (Wa). The values for the AFM surfaceroughness and the long-wavelength waviness may be obtained according tothe methods described in the Examples set forth below. The determinationis made on a substrate at a portion used for magnetic recording.Typically, the determination is made on a portion excluding 10% of theportions in a radial direction from each of the inner circumference andthe outer circumference of the substrate.

The combination of the “AFM surface roughness” determined by aninteratomic force microscope having a wavelength shorter than theconventionally determined roughness and “long-wavelength waviness”having a wavelength of 0.5 mm or more, which is longer than theconventional as the evaluation axes for the surface properties of thesubstrate for a magnetic disk seems to be surprising. Although notwanting to be limited by theory, these two parameters are presumed to beuseful as guidelines for the evaluation of the surface properties for ahigh recording density for the following reasons. High-speed recordingis to be achieved when the recording area per single bit is made smallerwith the high recording density. Therefore, the stability of the signalsascribed to the follow-up property of the magnetic head for thesubstrate has become important. The long-wavelength waviness may bedetermined by an optical instrument represented by “Zygo New View”commercially available from Canon Sales, Inc.

The substrate for a magnetic disk preferably has an AFM surfaceroughness of 0.03 nm or more and 0.2 nm or less, more preferably 0.03 nmor more and 0.15 nm or less, and even more preferably 0.03 nm or moreand 0.12 nm or less. Also, the substrate for a magnetic disk preferablyhas a long-wavelength waviness of 0.05 nm or more and 0.3 nm or less,more preferably 0.05 nm or more and 0.25 nm or less, and even morepreferably 0.05 nm or more and 0.2 nm or less.

The substrate for a magnetic disk of the present invention refers tothose which are used as substrates for media for magnetic recording,such as hard disks. Concrete examples of the substrate for a magneticdisk representatively include a substrate made of an aluminum alloyplated with Ni—P alloy; a substrate made of glass or glassy carbon,instead of the aluminum alloy, and plated with Ni—P thereon; a substratecoated with various metallic compounds by plating or deposition, insteadof the substrate plated with Ni—P; and the like.

The substrate for a magnetic disk of the present invention, havingpreferred surface properties that the substrate has a long-wavelengthwaviness of 0.05 nm or more and 0.3 nm or less, and an AFM surfaceroughness of 0.05 nm or more and 0.2 nm or less, has a high recordingdensity. Therefore, the substrate may be suitably used for a hard diskhaving a recording density of 50 G bits or more per square inch,preferably 80 G bits or more per square inch, more preferably 100 G bitsor more per square inch.

2. Method for Manufacturing Substrate for Magnetic Disk

A method for manufacturing a substrate for a magnetic disk includes thesteps of:

-   (a) polishing a substrate with a polishing composition (polishing    composition A) containing alumina abrasives having an average    particle size of from 0.05 to 0.5 μm, and an oxidizing agent; and-   (b) polishing the substrate obtained by the process of step (a) with    a polishing composition (polishing composition B) containing silica    particles having an average particle size of from 0.005 to 0.1 μm.

In order to manufacture the substrate for a magnetic disk having thepreferred surface properties of a long-wavelength waviness of 0.05 nm ormore and 0.3 nm or less, and an AFM surface roughness of 0.03 nm or moreand 0.2 nm or less, it is necessary that the long-wavelength waviness issufficiently reduced in the polishing step (step (a)) which is carriedout before step (b). Specifically, it is effective that thelong-wavelength waviness is sufficiently reduced in step (a) bypolishing with the polishing composition containing the aluminaabrasives having an average particle size of from 0.05 to 0.5 μm and theoxidizing agent, while preferably increasing the polishing rate. It ispreferable that step (b) is carried out in a final polishing step.

As the alumina abrasives usable in the present invention, the aluminaabrasives having a purity as alumina of 95% or more, more preferably 97%or more, and even more preferably 99% or more are preferable, from theviewpoints of reducing the long-wavelength waviness, reducing the AFMsurface roughness and increasing the polishing rate. The aluminaabrasives include a-alumina, intermediate alumina and compositesthereof. The intermediate alumina refers to alumina other than the 60-alumina, and concrete examples thereof include γ-alumina, δ-alumina,θ-alumina, η-alumina, κ-alumina, and composites thereof.

The average particle size of alumina abrasives is preferably 0.5 μm orless, more preferably 0.4 μm or less, even more preferably 0.3 μm orless, and even more preferably 0.25 μm or less, from the viewpoint ofreducing the long-wavelength waviness. The average particle size ispreferably 0.05 μm or more, more preferably 0.08 μm or more, even morepreferably 0.1 μm or more, and even more preferably 0.12 μm or more,from the viewpoint of increasing the polishing rate. Accordingly, inorder to achieve both the reduction of long-wavelength waviness andincrease in the polishing rate in a good balance, the average particlesize of alumina abrasives is preferably from 0.05 to 0.5 μm, morepreferably from 0.08 to 0.4 μm, even more preferably from 0.1 to 0.3 μm,and even more preferably from 0.12 to 0.25 μm. The average particle sizemay be determined as a volume-average particle size by using a laserdiffraction method.

The specific surface area of the alumina abrasives as determined by theBET method is as follows. The α-alumina has a specific surface area ofpreferably from 0.1 to 50 m²/g, more preferably from 1 to 40 m²/g, andeven more preferably from 2 to 20 m²/g, and the intermediate alumina hasa specific surface area of preferably from 30 to 300 m²/g, and morepreferably from 50 to 200 m²/g, from the viewpoint of reducing thelong-wavelength waviness.

As alumina abrasives, a mixture of the a-alumina and the intermediatealumina is effective, from the viewpoints of increasing the polishingrate and reducing the long-wavelength waviness. In this case, the weightratio of the α-alumina to the intermediate alumina,α-alumina/intermediate alumina, is preferably from 99/1 to 30/70, morepreferably from 97/3 to 40/60, even more preferably from 95/5 to 50/50,and even more preferably from 93/7 to 55/45.

The content of the alumina abrasives is preferably 0.05% by weight ormore, more preferably 0.1% by weight or more, even more preferably 0.5%by weight or more, and even more preferably 1% by weight or more, of thepolishing composition A, from the viewpoints of increasing the polishingrate and reducing the long-wavelength waviness. The content ispreferably 40% by weight or less, more preferably 30% by weight or less,even more preferably 20% by weight or less, and even more preferably 10%by weight or less, from the viewpoints of the surface qualities such assurface scratches, and economic advantages. Specifically, from bothviewpoints, the content of the alumina abrasives is preferably from 0.05to 40% by weight, more preferably from 0.1 to 30% by weight, even morepreferably from 0.5 to 20% by weight, and even more preferably from 1 to10% by weight, of the polishing composition A.

The polishing composition A contains an oxidizing agent. It is effectivefor obtaining the long-wavelength waviness and the polishing rate on anactual manufacturing level which are necessary for a magnetic disksubstrate usable for a hard disk having a high recording density,especially having a recording density of 50 G bits or more per squareinch. Although not wanting to be limited by theory, while the details ofthe functional mechanism for polishing may not be certain, it ispresumed that since an oxidizing agent is added, the oxidizing agentchanges the substrate surface to a state that the polishing effect ofthe alumina abrasives may be satisfactorily exhibited.

The oxidizing agent usable in the present invention includes a peroxide,a nitric acid compound, an oxidizable metallic compound, and the like.The peroxide includes hydrogen peroxide; peroxides of alkali metals orthe alkaline earth metals, such as sodium peroxide, potassium peroxide,calcium peroxide, barium peroxide and magnesium peroxide;peroxocarbonates, such as sodium peroxocarbonate and potassiumperoxocarbonate; peroxosulfuric acids and salts thereof, such asammonium peroxodisulfate, sodium peroxodisulfate, potassiumperoxodisulfate and peroxomonosulfuric acid; peroxonitric acids andsalts thereof, such as peroxonitric acid, sodium peroxonitrate andpotassium peroxonitrate; peroxophosphoric acids and salts thereof, suchas sodium peroxophosphate, potassium peroxophosphate and ammoniumperoxophosphate; peroxoborate, such as sodium peroxoborate and potassiumperoxoborate; halogeno-acids and salts thereof, such as potassiumperchlorate, chloric acid, sodium hypochlorite, sodium periodate,potassium periodate, iodic acid and sodium iodate; and percarboxylicacids and salts thereof, such as peracetic acid, performic acid andperbenzoic acid. The nitric acid compound includes nitric acid andnitrates such as sodium nitrate and potassium nitrate. The oxidizablemetallic compound includes iron (III) chloride; iron (III) sulfate; iron(III) citrate; iron (III) salt of EDTA, peroxochromates, such aspotassium peroxochromate and sodium peroxochromate; permanganates, suchas potassium permanganate and sodium permanganate. The peroxide ispreferable, from the viewpoints of increasing the polishing rate andreducing the long-wavelength waviness and from the viewpoints of easyhandling such as availability and water-solubility, and environmentalfriendliness. Among them, hydrogen peroxide, peroxosulfates or saltsthereof, halogeno-acids or salts thereof are more preferable, andhydrogen peroxide is even more preferable. In addition, these oxidizingagents may be used alone or in admixture of two or more kinds.

The content of the oxidizing agent is preferably 0.002% by weight ormore, more preferably 0.005% by weight or more, even more preferably0.007% by weight or more, and even more preferably 0.01% by weight ormore, of the polishing composition A, from the viewpoints of increasingthe polishing rate and reducing the long-wavelength waviness. Also, thecontent is preferably 20% by weight or less, more preferably 15% byweight or less, even more preferably 10% by weight or less, and evenmore preferably 5% by weight or less, from the viewpoints of the surfacequality and the economic advantages. Specifically, the content of theoxidizing agent is preferably from 0.002 to 20% by weight, morepreferably from 0.005 to 15% by weight, even more preferably from 0.007to 10% by weight, and even more preferably from 0.01 to 5% by weight, ofthe polishing composition A.

Water usable in the polishing composition A is used as a medium. Thecontent is preferably from 50 to 99% by weight, more preferably from 60to 97% by weight, and even more preferably from 70 to 95% by weight,from the viewpoint of effectively polishing the object to be polished.

It is preferable that the polishing composition A further contains anacid, from the viewpoints of increasing the polishing rate and reducingthe waviness. As the acid, a pK1 of the acid is preferably 7 or less,more preferably 5 or less, even more preferably 3 or less, and even morepreferably 2 or less. Here, the pK1 refers to a logarithmic value of aninverse of a first acid dissociation constant at 25° C. The pK1 of eachcompound is listed, for instance, in Kagaku Binran (Kiso-hen) II, FourthRevision, pp. 316-325 (Edit. by Nippon Kagakukai), and the like. As theacid, an inorganic acid and an organic acid may be used. The inorganicacid includes mineral acids, such as nitric acid, sulfuric acid,sulfurous acid, hydrochloric acid, perchloric acid, phosphoric acid,pyrophosphoric acid, polyphosphoric acid, phosphonic acid, phosphinicacid and amide sulfuric acid. The organic acid includes monocarboxylicacids, such as formic acid, acetic acid, glycolic acid, lactic acid,propanoic acid, hydroxypropanoic acid, butyric acid, benzoic acid, andglycine; polycarboxylic acids, such as oxalic acid, succinic acid,glutaric acid, adipic acid, maleic acid, fumaric acid, itaconic acid,malic acid, tartaric acid, citric acid, isocitric acid, phthalic acid,nitrilotriacetic acid and ethylenediaminetetraacetic acid; alkylsulfonicand arylsulfonic acids, such as methanesulfonic acid andparatoluenesulfonic acid; alkylphosphoric acids, such as ethylphosphoricacid and butylphosphoric acid; organic phosphonic acids, such asphosphonohydroxyacetic acid, hydroxyethylidene-1,1-diphosphonic acid,phosphonobutane tricarboxylic acid and ethylenediaminetetramethylenephosphonic acid. Among them, the polyvalent acids are preferable, morepreferably polyvalent mineral acids, polycarboxylic acids and organicpolyphosphonic acids, and even more preferably polyvalent mineral acidsand polycarboxylic acids, from the viewpoints of increasing thepolishing rate and reducing the long-wavelength waviness. The polyvalentacid as referred to herein is an acid having two or more hydrogen atomsin its molecule, capable of generating hydrogen ions.

The acids may be used alone, and it is preferable to use the acids inadmixture of two or more kinds. Especially in the case of polishing amagnetic disk substrate, a metal ion of an object to be polished iseluted during polishing, so that the pH of the polishing composition isincreased, and that a high polishing rate cannot be obtained. In such acase, it is preferable to use a combination of an acid having a low pKaand an acid having a high pKa, in order to make the pH change smaller.When the polishing composition contains two or more kinds of the acidsas mentioned above, as the acid having a low pKa, it is preferable touse a mineral acid, such as nitric acid, sulfuric acid, phosphoric acidor polyphosphoric acid, or an organic phosphonic acid, in considerationsof increase in the polishing rate, reduction in the waviness, andavailability. On the other hand, as the acid having a high pKa, anorganic polycarboxylic acid, such as acetic acid, succinic acid, malicacid, tartaric acid or citric acid is preferable, from the sameviewpoints.

The content of the acid is preferably from 0.002 to 20% by weight, morepreferably from 0.005 to 15% by weight, even more preferably from 0.007to 10% by weight, and even more preferably from 0.01 to 5% by weight, ofthe polishing composition A, from the viewpoints of increasing thepolishing rate and reducing the long-wavelength waviness.

In addition, there can be added one or more components such as aninorganic salt, a thickener, an anticorrosive agent or a basic substanceto the polishing composition A, as occasion demands. Especially, theinorganic salt such as ammonium nitrate, ammonium sulfate, potassiumsulfate, nickel sulfate, aluminum sulfate or ammonium sulfamate has anauxiliary effect for increasing the polishing rate may be added. Thesecomponents may be used alone or in admixture of two or more kinds. Also,the content of the other component is preferably from 0.05 to 20% byweight, more preferably from 0.05 to 10% by weight, and even morepreferably from 0.05 to 5% by weight, of the polishing composition A,from the viewpoint of economic advantages. In addition, there can beadded one or more components, such as a disinfectant or an antibacterialagent, as occasion demands.

The polishing composition A may be prepared by adding or mixing theintended components by a given method.

It is preferable that the pH of the polishing composition A isappropriately determined depending upon the kinds of the object to bepolished and the required properties. For instance, the pH of thepolishing composition A is preferably from 0.1 to 6, more preferablyfrom 0.5 to 5, even more preferably from 1 to 4, and even morepreferably from 1 to 3, from the viewpoints of increase in the polishingrate and reduction in the long-wavelength waviness, and from theviewpoints of prevention of the corrosion of the processing machine andoperator safety. The pH may be adjusted by properly adding an inorganicacid, such as nitric acid or sulfuric acid; an organic acid, such as ahydroxycarboxylic acid, a polycarboxylic acid, an aminopolycarboxylicacid, an amino acid, or a metal salt or an ammonium salt thereof; or abasic substance, such as an aqueous ammonia, sodium hydroxide, potassiumhydroxide or an amine, in a desired amount.

In step (a), the substrate having reduced AFM surface roughness andlong-wavelength waviness may be manufactured at a polishing rate on anactual manufacturing level by a method including the steps of setting asubstrate with polishing platens to which a polishing cloth made of aporous organic polymer and the like is attached; feeding the polishingcomposition A to a surface to be polished; and moving the polishingplatens or the substrate, while applying pressure. In order tomanufacture a substrate for a magnetic disk having a high recordingdensity, especially a substrate for a magnetic disk having a recordingdensity of 50 G bits or more per square inch, after step (a), thesubstrate has an upper limit of the long-wavelength waviness ofpreferably 0.4 nm or less, more preferably 0.35 nm or less, even morepreferably 0.3 nm or less, and even more preferably 0.25 nm or less, andthe substrate has a lower limit of the long-wavelength waviness ofpreferably 0.05 nm or more, more preferably 0.1 nm or more. Otherconditions of step (a) such as a polishing machine, a polishingtemperature, a polishing rate, and a feed amount of the polishingcomposition A are not particularly limited.

In addition, one feature of the method for manufacturing a substrate fora magnetic disk in the present invention resides in that step (a) iscarried out prior to step (b).

Any polishing step carried out before step (b), preferably the finalpolishing step, may be carried out in a plurality of steps. In order toobtain a substrate for a magnetic disk for a hard disk having a highrecording density, especially a substrate for a magnetic disk for a harddisk having a high recording density of 50 G bits or more per squareinch, it is preferable that at least one of the steps prior to step (b)includes the step of polishing a substrate with a polishing compositioncontaining alumina abrasives having an average particle size of from0.05 to 0.5 μm and an oxidizing agent.

In the case where the polishing is carried out in plural steps, it ispreferable that the sizes of the abrasives are stepwise made smaller. Inaddition, these polishing steps may be carried out continuously in thesame polishing machine, or alternatively, separate polishing machines inorder to avoid admixing of the abrasives or the polishing compositionsused in the previous step. In the case where separate polishing machinesare used for each of the steps, it is preferable to clean the substrateafter each step.

In order to obtain a substrate for a magnetic disk having a highrecording density after step (b), it is desired that the polished amountof the substrate that has been polished before step (b) is performed is0.8 μm or more, preferably 1 μm or more, more preferably 1.2 μm or more,from the viewpoint of reduction of the long-wavelength waviness. Also,it is desired that this polished amount is 4 μm or less, preferably 3 μmor less, more preferably 2 μm or less, from the industrial viewpoint.

In addition, the polished amount of the substrate that has been polishedduring step (a) is preferably 0.2 μm or more, more preferably 0.3 μm ormore, and even more preferably 0.5 μm or more, from the viewpoint ofreduction of the long-wavelength waviness. Also, it is desired that thispolished amount is 4 μm or less, preferably 3 μm or less, morepreferably 2 μm or less, from the industrial viewpoint.

These polished amounts may be determined in accordance with the methodsdescribed in the Examples set forth below.

Furthermore, in consideration of an actual mass-production of themagnetic disk substrate, the polishing time of the substrate that hasbeen polished before the polishing step (b) is preferably 10 minutes orless, more preferably 8 minutes or less, and even more preferably 6minutes or less. The polishing rate is preferably 0.1 μm/min or more,more preferably 0.2 μm/min or more, even more preferably 0.25 μm/min ormore, and even more preferably 0.3 μm/min or more, from the sameviewpoints.

In the present invention, in step (b), preferably the final polishingstep, the substrate for the magnetic disk having the intended surfaceproperties can be preferably manufactured by polishing a substrate ofwhich long-wavelength waviness and AFM surface roughness are reduced toa given level in the previous polishing steps, with the polishingcomposition (polishing composition B) containing the silica particleshaving an average particle size of from 0.005 to 0.1 μm.

The silica particles include colloidal silica particles and fumed silicaparticles. Among them, colloidal silica particles are preferable. Thecolloidal silica particles obtained by the method including the step offorming the particles from an aqueous silicic acid are more preferable,from the industrial viewpoint. Also, as the silica particles,surface-modified silica particles may be used, from the viewpoints ofreducing AFM surface roughness, increasing the polishing rate andpreventing defects of the substrate.

The average particle size of the silica particles is preferably from0.005 to 0.1 μm, more preferably from 0.008 to 0.08 μm, and even morepreferably from 0.01 to 0.05 μm, from the viewpoints of reducing the AFMsurface roughness and long-wavelength waviness and increasing thepolishing rate. The average particle size may be determined by themethod described below. The photographs of the abrasive particlesobserved by a transmission electron microscope “JEM-200FX” commerciallyavailable from JEOL LTD. (magnification: 10000 to 50000) areincorporated into a personal computer as image data with a scannerconnected thereto. The projected area diameter of each particle isdetermined, using an analysis software “WinROOF” (commercially availablefrom MITANI CORPORATION). Considering such diameter as the diameter ofthe particles, the average particle size is determined based on thenumber basis by analyzing data using a spreadsheet software “EXCEL”(commercially available from Microsoft Corporation). The averageparticle size is expressed based on the number basis. Additionally, byusing the above-mentioned “EXCEL,” the particle size distribution dataof the particles on the volume basis may be also obtained by calculatingthe diameters of the particles into the volume of the particles. Thenumber to be observed is preferably at least 1000, more preferably 3000or more, and even more preferably 5000 or more.

As the silica particles, those particles of which 50% (% volume based)or more of the particles having a particle size of 0.05 μm or less, andmore preferably 0.04 μm or less are preferable, wherein the particlesize is determined by the above-mentioned method, from the viewpoint ofreducing the surface roughness and long-wavelength waviness. This %volume based on the particles may be determined by the method describedin Examples set forth below.

The content of the silica particles is preferably from 1 to 30% byweight, more preferably from 2 to 20% by weight, and even morepreferably from 3 to 10% by weight, of the polishing composition B, fromthe viewpoints of an increase in the polishing rate, and economicadvantages.

It is preferable that the oxidizing agent and the acid (and/or the saltthereof) is added and mixed to the polishing composition B usable instep (b), more preferably both the oxidizing agent and the acid areadded and mixed, from the viewpoints of reducing the long-wavelengthwaviness and the AFM surface roughness and increasing the polishingrate.

The oxidizing agent includes a peroxide, permanganic acid or a saltthereof, chromic acid or a salt thereof, nitric acid or a salt thereof,a peroxo acid and a salt thereof, an oxyacid or a salt thereof, a metalsalt of an acid, a sulfuric acid, and the like. As the peroxide, thesame peroxides as in the polishing composition A may be used, andespecially hydrogen peroxide, a peroxo acid or a salt thereof, and anoxyacid or salt thereof are preferable, and even more preferablyhydrogen peroxide, from the viewpoints of no deposition of a metal ionto the surface, a wide use and a low cost. These oxidizing agents may beused alone or in admixture of two or more kinds. Among these oxidizingagents, nitric acid or a salt thereof may be used as an acid or a saltthereof having a pK1 of 2 or less as described later.

The content of the oxidizing agent is preferably from 0.002 to 20% byweight, more preferably from 0.005 to 15% by weight, even morepreferably from 0.007 to 10% by weight, and even more preferably from0.01 to 5% by weight, of the polishing composition B, from theviewpoints of increasing the polishing rate, reducing the AFM surfaceroughness and long-wavelength waviness and reducing the surface defectssuch as pits or scratches, thereby improving the surface qualities, andfrom the viewpoint of economic advantages.

As the acid, the acid which is listed in polishing composition A may beused. Among them, the inorganic acid, the organic phosphonic acid andsalts thereof are preferable, from the viewpoints of increasing thepolishing rate, reducing the AFM surface roughness and long-wavelengthwaviness, and furthermore reducing microscratches. As the inorganic acidand the salt thereof, nitric acid, sulfuric acid, hydrochloric acid,perchloric acid and the salts thereof are preferable. As the organicphosphonic acid or the salt thereof,1-hydroxyethylidene-1,1-diphosphonic acid, aminotri(methylenephosphonicacid), ethylenediaminetetra(methylenephosphonic acid) anddiethylenetriaminepenta(methylenephosphonic acid) and salts thereof arepreferable. In view of pK1 of the acid, the acid and/or a salt thereofhaving a pK1 of 2 or less is even more preferable. The base of theseacids is not particularly limited, and the salts with the metalbelonging to the 1A group or with ammonium are preferable, from theviewpoint of reducing microscratches. These acids and the salts thereofmay be used alone or in admixture of two or more kinds.

The content of the acid is preferably from 0.1 to 10% by weight, morepreferably from 0.2 to 5% by weight, and even more preferably from 0.3to 3% by weight, of the polishing composition B, from the viewpoints ofreduction of the AFM surface roughness, increase in the polishing rate,and economic advantages.

In the polishing composition B, the balance is preferably water. Waterusable in the polishing composition B is used as a medium, and thecontent is preferably from 60 to 98% by weight, more preferably from 70to 97% by weight, and even more preferably from 80 to 96% by weight,from the viewpoints of reduction of the AFM surface roughness, increasein the polishing rate, and economic advantages.

In addition, there can be added one or more components, such as aninorganic salt, a thickener, an anticorrosive agent or a basicsubstance, to the polishing composition B, in the same manner as in thepolishing composition A, as occasion demands. Especially, an inorganicsalt, such as ammonium nitrate, ammonium sulfate, potassium sulfate,nickel sulfate, aluminum sulfate, or ammonium sulfamate, has anauxiliary effect of increasing the polishing rate. One or more of thesecomponents may be used alone or in admixture of two or more kinds. Also,the content of the components is preferably from 0.05 to 20% by weight,more preferably from 0.05 to 10% by weight, and even more preferablyfrom 0.05 to 5% by weight, of the polishing composition B, from theviewpoint of economic advantages. In addition, there can be added one ormore component, such as a disinfectant or an antibacterial agent, asoccasion demands.

The concentration of each component of the polishing composition A andthe polishing composition B is a concentration at which polishing ispreferably carried out. The concentration may be the concentrationduring the preparation of the composition. In many cases, thecomposition is usually prepared as a concentrate, which is dilutedbefore use or upon use.

The polishing composition B may be prepared by adding or mixing theintended components by a given method. It is preferable that the pH ofthe polishing composition B is appropriately determined depending uponthe kinds of the object to be polished and the required properties. Forinstance, the pH of the polishing composition B is preferably from 0.1to 6, more preferably from 0.5 to 5, even more preferably from 1 to 4,and even more preferably from 1 to 3, from the viewpoints of increase inthe polishing rate and reduction in the long-wavelength waviness, andfrom the viewpoints of prevention of the corrosion of the processingmachine and operator safety. The pH may be adjusted by properly addingan inorganic acid, such as nitric acid or sulfuric acid; an organicacid, such as a hydroxycarboxylic acid, a polycarboxylic acid, anaminopolycarboxylic acid, an amino acid, or a metal salt or an ammoniumsalt thereof; or a basic substance, such as an aqueous ammonia, sodiumhydroxide, potassium hydroxide or an amine, in a desired amount.

In step (b), in the same manner as the above-mentioned step (a), thesubstrate having reduced AFM surface roughness and long-wavelengthwaviness may be manufactured at a polishing rate at the level used foran actual mass-production using the polishing process, including thesteps of setting a substrate between polishing platens to which apolishing pad made of a porous organic polymer is attached; feeding apolishing composition B to the surface of a substrate to be polished;and moving the polishing platens and the substrate while applyingpressure.

As mentioned-above, by using the method for manufacturing a substratefor a magnetic disk including step (a) and step (b), step (a) includingthe step of polishing a substrate with polishing composition Acontaining alumina abrasives having an average particle size of from0.05 to 0.5 μm, and an oxidizing agent, is carried out before step (b),thereby adjusting a long-wavelength waviness to preferably 0.05 nm ormore and 0.4 nm or less, more preferably 0.05 nm or more and 0.35 nm orless, even more preferably 0.1 nm or more and 0.3 nm or less, and evenmore preferably 0.1 nm or more and 0.25 nm or less. Thereafter, step(b), preferably the final polishing step, including the step ofpolishing the resulting substrate with the polishing composition Bcontaining silica particles having an average particle size of from0.005 to 0.1 μm is carried out, whereby a substrate for a magnetic diskfor a high recording density, the substrate having an AFM surfaceroughness of preferably 0.03 nm or more and 0.2 nm or less, morepreferably 0.03 nm or more and 0.15 nm or less, and even more preferably0.03 nm or more and 0.12 nm or less, and a long-wavelength waviness ofpreferably 0.05 nm or more and 0.3 nm or less, preferably 0.05 nm ormore and 0.25 nm or less, more preferably 0.05 nm or more and 0.2 nm orless, may be manufactured.

Since the obtained substrate for the magnetic disk has extremelyexcellent surface qualities, the substrate may be suitably used as asubstrate for manufacturing a hard disk having a high recording density.

EXAMPLES

The following examples further describe and demonstrate embodiments ofthe present invention. The examples are given solely for the purposes ofillustration and are not to be construed as limitations of the presentinvention.

Examples 1 to 4 and Comparative Examples 1 to 5

1. Preparation of Polishing Composition A and Polishing Composition B

Alumina abrasive grains, an oxidizing agent and various agents wereadded as shown in Table 1, to give a polishing composition A which wasto be used in step (a). In addition, the colloidal silica and variousagents were added as shown in Table 2, to give a polishing composition Bwhich was to be used in step (b). In both the polishing compositions Aand B, the balance was water.

2. Polishing Process of Step (a)

A substrate (long-wavelength waviness: 1.6 nm, as determined by Zygo NewView 200) made of a Ni—P plated, aluminum alloy (area of both sides:131.94 cm³, Ni—P plating density: 8.4 g/cm³), having a thickness of 1.27mm, and a diameter of 3.5 inch (95 mm) was polished using a double-sidedprocessing machine under the following setting conditions, to give anobject to be polished made of Ni—P plated aluminum alloy substrateusable as a substrate for a magnetic recording medium.

The setting conditions for the double-sided processing machine are asfollows.

<Setting Conditions for Double-Sided Processing Machine>Double-sidedprocessing machine: Model 9B, commercially available from SPEEDFAM CO.,LTD.

Processing pressure: 9.8 kPa

Polishing pad: “H9900” (trade name, commercially available from FUJIBO)

Rotational speed of a platen: 50 r/min

Feeding amount for a polishing composition: 100 ml/min

Polishing time period: shown in Table 1

Number of substrates introduced: 10

3. Polishing Process of Step (b)

The substrate obtained by various polishing processes before the finalpolishing was polished under the following setting conditions for thedouble-sided processing machine, to give an object to be polished madeof Ni—P plated aluminum alloy substrate usable as a substrate for amagnetic recording medium.

The setting conditions for the double-sided processing machine are asfollows.

<Setting Conditions for Double-Sided Processing Machine>

Double-sided processing machine: Model 9B, commercially available fromSPEEDFAM CO., LTD.

Processing pressure: 7.8 kPa

Polishing pad: “Belatrix N0058” (trade name), commercially availablefrom Kanebo, LTD.)

Rotational speed of a platen: 35 r/min

Feeding amount for a polishing composition: 100 ml/min

Polishing time period: shown in Table 2

Number of substrates introduced: 10

4. Evaluation Methods

(1) Polished Amount (Removal) and Polishing Rate (Removal Rate)

Each of the substrates before and after polishing were weighed with abalance (“BP-210S” commercially available from Sartorius), and a changein weight was obtained for each substrate. An average for the polishedamount of 10 substrates was taken and defined as the weight reduction.The polished amount was obtained from the following equation. Also, thevalue obtained by dividing the polished amount by the polishing time wasdefined as a polishing rate. Here, the polished amount is expressed interms of the polished-off thickness of the substrate.Polished Amount (μm)=[[Weight Before Polishing (g)]−[Weight AfterPolishing (g)]]/[[Area of Both Sides of Substrate (cm²)]×[Ni—P PlatingDensity (g/cm³)]]×10000Polishing Rate (μm/min)=[Polished Amount (μm)]/[Polishing Time (min)](2) Long-Wavelength Waviness

The long-wavelength waviness for each of the polished substrate wasdetermined under the following conditions.

-   -   Device: Zygo New View 200 commercially available from Canon        Sales, Inc.    -   Lens: Magnification, 2.5 times, Michelson    -   Zooming Ratio: 0.5    -   Camera: 320×240 Normal    -   Remove: Cylinder    -   Filter type: FFT Fixed Band Pass Filter High Wavelength 0.5 mm        Filter Low Wavelength 5 mm    -   Area: 4.33 mm×5.77 mm

The determination area was selected so that the center of thedetermination area is along a central line drawn at an equidistance fromthe inner circumference and the outer circumference of the disk, andthat the arc direction of the area is parallel to a tangential directionof the circular disk.

Determinations were taken on five areas on each of the front and backsides per one disk evenly in a circumferential direction, and an averageof the determinations was defined as the “long-wavelength waviness” ofthe disk.

(3) AFM Surface Roughness

The AFM surface roughness for each of the polished substrate wasdetermined under the following conditions.

-   -   Device: Interatomic Force Microscope “M5E” commercially        available from Veeco    -   Cantilever: UL20B    -   Mode: Non-Contact    -   Scan Rate: 1.0 Hz    -   Scan area: 5×5 μm    -   Diameter of Profilometer: 10 nm

The determination area was selected in the same manner as that oflong-wavelength waviness. Determinations were taken on five areas oneach of the front and back sides per one disk evenly in acircumferential direction, and an average of the determinations wasdefined as “AFM surface roughness” of the disk. TABLE 1 SurfacePolishing Step before Final Polishing Working (step (a)) PropertiesAluminia Abrasive Grains Other Additive After step (a) IntermediateLong- α-Alumina % Alumina % % % Polishing Polished Polishing Wavelength(Average by (Average by by Acid and by Time Amount Rate WavinessParticle Size wt. Particle Size) wt. Compound wt. the like wt. pH (min.)(μm) (μm/min) (nm) Ex. No. Ex. a α-Alumina 4 θ-Alumina 1 Hydrogen 0.8Citric Acid 1 2.0 4 1.5 0.38 0.24 (0.2 μm) (0.2 μm) Peroxide SulfuricAcid 0.2 Ammonium 2 Sulfate Ex. b α-Alumina 4 θ-Alumina 1 Hydrogen 0.8Citric Acid 1 1.8 4 1.8 0.45 0.25 (0.3 μm) (0.2 μm) Peroxide Sulfuric0.2 Acid Ex. c α-Alumina 4 θ-Alumina 1 Hydrogen 0.8 Sulfuric Acid 0.51.4 4 1.4 0.35 0.28 (0.3 μm) (0.2 μm) Peroxide Comp. Ex. No. Comp.α-Alumina 10 — — Sulfonic 0.5 4 3 1.4 0.35 Ex. a   (1 μm) Acid α-Alumina10 — — Sulfonic 0.5 4 1 0.08 0.08 0.65 (0.4 μm) Acidα-Alumina (average particle size: 0.2 μm): purity 99.9%, specificsurface area 18 m²/gα-Alumina (average particle size: 0.3 μm): purity 99.9%, specificsurface area 17 m²/gα-Alumina (average particle size: 0.4 μm): purity 99.9%, specificsurface area 15 m²/gα-Alumina (average particle size: 1.0 μm): purity 99.9%, specificsurface area 13 m²/gθ-Alumina (average particle size: 0.2 μm): purity 99.9%, specificsurface area 120 m²/gIn Comparative Example a, a substrate was polished with the polishingcomposition and conditions in the upper row and thereafter polished withthe polishing composition and conditions in the lower row.

TABLE 2 Surface Properties Step Final Polishing Working (step (b)) ofFinal Substrate Before Abrasive Long- AFM Final Grains % % % PolishingWavelength Surface Polishing (Average Particle by Oxidizing by by TimeWaviness Roughness Working Size) wt. Agent wt. Other Additive wt. pH(min.) (nm) (nm) Ex. No. Ex. 1 Ex. a Colloidal Silica 7 Hydrogen 0.4Hydroxyethylidene 1.5 1.5 4 0.17 0.12 (0.02 μm) Peroxide DiphosphonicAcid Ex. 2 Ex. b Colloidal Silica 7 Hydrogen 0.4 Hydroxyethylidene 1.51.5 4 0.19 0.13 (0.02 μm) Peroxide Diphosphonic Acid Ex. 3 Ex. cColloidal Silica 7 Hydrogen 0.4 Hydroxyethylidene 1.5 1.5 4 0.19 0.14(0.02 μm) Peroxide Diphosphonic Acid Ex. 4 Ex. c Colloidal Silica 7Hydrogen 0.4 Hydroxyethylidene 1.5 1.5 4 0.19 0.15 (0.02 μm) PeroxideDiphosphonic Acid Comp. Ex. No. Comp. None Colloidal Silica 7 Hydrogen0.4 Hydroxyethylidene 1.5 1.5 8 0.64 0.25 Ex. 1 (0.02 μm) PeroxideDiphosphonic Acid Comp. Ex. a Not polished 0.24 1.4 Ex. 2 Comp. Ex. aColloidal Silica 7 Hydrogen 0.4 Hydroxyethylidene 1.5 1.5 4 0.22 0.34Ex. 3 (0.12 μm) Peroxide Diphosphonic Acid Comp. Comp. Colloidal Silica7 Hydrogen 0.4 Hydroxyethylidene 1.5 1.5 4 0.38 0.21 Ex. 4 Ex. a (0.02μm) Peroxide Diphosphonic Acid Comp. Comp. Colloidal Silica 10 — —Sodium Hydroxide 2 10 3 0.55 0.35 Ex. 5 Ex. a (0.05 μm)Colloidal Silica (average particle size: 0.02 μm): volume-based particlesize at 50%: 0.035 μmColloidal Silica (average particle size: 0.05 μm): volume-based particlesize at 50%: 0.07 μmColloidal Silica (average particle size: 0.12 μm): volume-based particlesize at 50%: 0.17 μm

It can be seen from the results shown in Tables 1 and 2 that all of thesubstrates for magnetic disks obtained in Examples 1 to 4 had veryexcellent surface properties of a long-wavelength waviness of 0.05 nm ormore and 0.3 nm or less and an AFM surface roughness of 0.03 nm or moreand 0.2 nm or less.

The substrate for a magnetic disk of the present invention can besuitably used in the manufacture of a hard disk having a high recordingdensity. Especially, a hard disk having a high recording density of 50 Gbits or more per square inch may be industrially manufactured.

The present invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A method for manufacturing a substrate for a magnetic disk,comprising the steps of: (a) polishing a substrate with a polishingcomposition A comprising alumina abrasives having an average particlesize of from 0.05 to 0.5 μm, and an oxidizing agent; and (b) polishingthe substrate with a polishing composition B comprising silica particleshaving an average particle size of from 0.005 to 0.1 μm.
 2. The methodaccording to claim 1, wherein the alumina abrasives comprise anα-alumina and an intermediate alumina, wherein the weight ratio of theα-alumina to the intermediate alumina, α-alumina/intermediate alumina,is from 99/1 to 30/70.
 3. The method according to claim 1, wherein theoxidizing agent is a peroxide.
 4. The method according to claim 1,wherein the polishing composition A further comprises an acid.
 5. Themethod according to claim 3, wherein the polishing composition A furthercomprises an acid.
 6. The method according to claim 1, wherein thesubstrate obtained after step (a) has surface properties of along-wavelength waviness of 0.05 nm or more and 0.4 nm or less.
 7. Themethod according to claim 4, wherein the substrate obtained after step(a) has surface properties of a long-wavelength waviness of 0.05 nm ormore and 0.4 nm or less.
 8. The method according to claim 1, wherein thepolished amount of the substrate that has been polished before step (b)is performed is 0.8 μm or more, and the polished amount of the substratethat has been polished during step (a) is 0.2 μm or more, wherein thepolished amount is expressed in terms of the polished-off thickness ofthe substrate.
 9. The method according to claim 4, wherein the polishedamount of the substrate that has been polished before step (b) isperformed is 0.8 μm or more, and the polished amount of the substratethat has been polished during step (a) is 0.2 μm or more, wherein thepolished amount is expressed in terms of the polished-off thickness ofthe substrate.
 10. The method according to claim 1, wherein thesubstrate obtained after step (b) has surface properties of along-wavelength waviness of 0.05 nm or more and 0.3 nm or less, and anAFM surface roughness of 0.03 nm or more and 0.2 nm or less.
 11. Themethod according to claim 4, wherein the substrate obtained after step(b) has surface properties of a long-wavelength waviness of 0.05 nm ormore and 0.3 nm or less, and an AFM surface roughness of 0.03 nm or moreand 0.2 nm or less.
 12. The method according to claim 6, wherein thesubstrate obtained after step (b) has surface properties of along-wavelength waviness of 0.05 nm or more and 0.3 nm or less, and anAFM surface roughness of 0.03 nm or more and 0.2 nm or less.
 13. Themethod according to claim 1, wherein the substrate that is to besubjected to step (b) has surface properties of a long-wavelengthwaviness of 0.05 nm or more and 0.4 nm or less.
 14. The method accordingto claim 4, wherein a substrate that is to be subjected to step (b) hassurface properties of a long-wavelength waviness of 0.05 nm or more and0.4 nm or less.
 15. A substrate for a magnetic disk obtained by themethod of claim
 1. 16. The substrate according to claim 15, wherein thesubstrate has the following surface properties: a long-wavelengthwaviness of 0.05 nm or more and 0.3 nm or less, and an AFM surfaceroughness of 0.03 nm or more and 0.2 nm or less.
 17. The substrateaccording to claim 15, wherein the substrate after step (a) has surfaceproperties of a long-wavelength waviness of 0.05 nm or more and 0.4 nmor less.
 18. The substrate according to claim 15, wherein the substratethat is to be subjected to step (b) has surface properties of along-wavelength waviness of 0.05 nm or more and 0.4 nm or less.
 19. Asubstrate for a magnetic disk having the following surface properties: along-wavelength waviness of 0.05 nm or more and 0.3 nm or less, and anAFM surface roughness of 0.03 nm or more and 0.2 nm or less.
 20. Thesubstrate according to claim 19, wherein the substrate is used for ahard disk having a recording density of 50 G bits or more per squareinch.